A constant problem in the packaging of commercial products, particularly those for human consumption, is how to protect the integrity of these products from damage caused by exposure to moisture. Oriented polypropylene (OPP) is known for its inherent moisture barrier properties. However, certain applications require even greater resistance to water vapor transmission to increase shelf-life. It is known to provide enhanced moisture barrier capability by techniques such as coating with polyvinylidene chloride (PVDC) as taught in U.S. Pat. Nos. 5,057,177; 5,019,447; and 4,961,992. However, such methods suffer from the need to provide a separate coating of the OPP. More significantly, efficient recycling of PVDC coated OPP can be difficult. Accordingly, it would be desirable to provide an OPP film of high moisture barrier properties which does not require a difficult to recycle separate coating. It would also be desirable for such high moisture barrier OPP film to be compatible with, or capable of, modification by coating processes. Such modifications include: improving oxygen barrier by applying PVOH coating; enhancing flavor and aroma protection by applying acrylic coating; improving machineability and capability for use in high speed horizontal form and fill applications by applying low temperature sealable coatings. U.S. Pat. No. 5,230,963 discloses enhancing oxygen barrier of films by a method involving a coating of poly(vinyl)alcohol.
It would also be useful to provide an OPP film of high moisture barrier properties which is susceptible of coextrusion with heat sealable layers to provide an ABA coextruded film.
U.S. Pat. Nos. 3,278,646 to Lambert and 4,230,767 to Isaka, et al. disclose oriented polypropylene films which contain terpene polymer which is primarily added to improve heat seal properties.
U.S. Pat. No. 3,937,762 to Nahmias, et al. discloses a resinous blend composition for films comprising a polypropylene containing a minor amount of terpene, e.g., alpha-pinene, beta-pinene or beta-phellandrene.
U.S. Pat. No. 5,128,183 to Buzio discloses films comprising a ternary mixture of (1) isotactic polypropylene; (2) high density polyethylene; and (3) a glassy, amorphous low molecular weight resin, e.g. terpene polymers. The resin is added at levels above 5 wt % to exert a compatibilizing effect on the blend of polypropylene and polyethylene providing a transparent film. The polyethylene component is added to facilitate mixing and homogeneity of the product.
Although it is known to add terpene polymers to certain polypropylenes for the purpose of improving heat seal, their use for improving moisture barrier characteristics in conventional OPP films has required relatively large amounts, say, 10 to 15 wt%, in order to be effective.
It is an object of the present invention to provide a packaging film material which exhibits high moisture barrier properties by blending low levels of terpene polymer into polypropylene core resin.
It is a further object of the present invention to provide an OPP film of high moisture barrier properties whose properties are further enhanced by the use of heat sealable layers, and/or coatings which improve oxygen barrier, flavor and aroma protection, machinability and applicability to high speed horizontal form and fill applications.